Investigation on the Anomalous Weakening of Diurnal Tides in the Mesosphere-Lower Thermosphere

Variations in tidal characteristics assume importance as far as the atmosphere-ionosphere coupling is concerned. The present study examines short-term variabilities in diurnal tidal characteristics, using one decade (2006-2015) of wind measurements in the mesosphere-lower thermosphere (MLT) region over a near equatorial location, Thumba (8.5 degrees N, 77 degrees E). The study presents two distinct events of anomalous tidal weakening, with decrease in monthly mean tidal amplitude by -38 ms(-1) (-26 ms(-1)) in February 2010 and -17 ms(-1) (-8 ms(-1)) in January 2012 at 98 km (91 km). These events are observed to be coinciding with the occurrence of strong quasi-two-day wave (QTDW). Compared to pre-event conditions, decrease (increase) in tidal (QTDW) amplitudes at 91 km is -13.03 ms(-1) (34.97 ms(-1)) and -15.65 ms(-1) (52.81 ms(-1)) during the first and second event, respectively. However, causative mechanisms are found to be different for both the events. While the former event is primarily due to parametric excitation of phase locked 2-day waves by diurnal tide, the latter one is mainly due to non-linear interaction between tide and QTDW and subsequent generation of secondary waves. While QTDW phase is relatively stable during the former event, it shows steep temporal variations during the latter one. Changes in background wind conditions are also found to be more conducive for weakening of diurnal tide during the former event. These anomalous changes in tidal characteristics would have significant impacts on ionospheric processes. The significance of this study lies in providing observational evidence for the existence of different pathways through which QTDW affects tidal amplitudes over the low-latitude MLT region.